A wide variety of pigments are used in paint and coating compositions, each with its own unique structure and properties. Consequently, each different pigment tends behave slightly differently in coating compositions. The different behaviors of the pigments, such as their effects on rheology and viscosity of the coating composition, require special consideration in formulating and manufacturing the coating composition. Further, care must be taken to maintain a stable dispersion of the pigment in the coating composition to prevent settling and color deterioration.
In general, industrial and automotive coatings are pigmented using dry pigments that have been pre-dispersed in a liquid pigment dispersion. The pigment dispersions are prepared by shearing the dry, agglomerated pigment in a liquid resin system to break apart the pigment agglomerates to the primary pigment aggregate particles and to intimately associate the resin with the surface of the pigment particles. These actions are necessary for proper color development of the pigment in the coating. These pigment dispersions can still exhibit certain problems, including settling and seeding during storage and interactions that affect the physical properties of the paint such as rheology and viscosity.
Dispersions of encapsulated pigments have been suggested for certain uses. GB 1 156 653, for example, discloses dispersions of coated pigment particles prepared by steps of grinding or milling the pigment in a solution of polymer and stabilizer, modifying the dispersion to change the organic liquid to a non-solvent, and then encapsulating the disperse particles by a polymer formed by dispersion polymerization of a second polymer that is insoluble in the organic liquid. The change of the organic liquid may be carried out by adding a liquid that is a non-solvent for the polymer or, if the liquid is a mixture of solvent and non-solvents, by removing part of the solvent component, e.g. by evaporation or partition. The process described in GB 1 156 653 is difficult and complicated. The change in the liquid medium from an organic solvent to an organic nonsolvent not only is a step where the pigment may become destabilized if conditions are not carefully controlled, but the step uses a large amount of organic liquids. Thus, the process does not lend itself to modern coatings that limit organic liquids through high solids coatings or aqueous coatings formulations. While the contemporary coatings of GB 1 156 653 in the mid 1960s may have had 83 to 85% by weight volatile organic compounds, today's coatings generally have less than 30% by weight volatile organic compounds. Finally, using pigment dispersed in organic liquid to pigment coatings introduces storage and manufacturing problems that traditional pigment pastes do not have, not the least of which is the storage of a much larger amount of organic liquid with the pigment and introduction of these large amount of organic liquid into the coating composition along with the pigment.
U.S. Pat. No. 3,849,152 describes encapsulating certain inorganic pigments by dispersing the pigments in an organic solvent, polymerizing a polysiloxane polymer in the solvent, and then spray-drying the dispersion to obtain encapsulated pigment particles. The inorganic pigment may have hydroxyl groups reactive with the polysiloxane polymer.
U.S. Pat. No. 3,826,670 describes preparing an encapsulated organic pigment with an intermediate layer of an ionically crosslinked polymer salt, such as a polyvalent metal salt of a polymer of an α,β-ethylenically unsaturated acid, and an outer skin of a dense, hydrated, amorphous oxide of silicon, zirconium, or titanium. The encapsulated pigment is reported to be chemically inert with excellent dispersibility.
U.S. Pat. No. 4,771,086 describes suspending in aqueous medium a pigment (e.g., TiO2), water-insoluble monomer, and water-soluble, nonionic surfactant. The monomer is addition polymerized using an initiator. The suspension of encapsulated pigment can be used as a paint with improved hiding and color compared with conventional latex paints. U.S. Pat. No. 3,849,152 discloses pigment slurried in, e.g., acetone, hexane, or trichloroethylene, the pigment being completely covered with a polysiloxane liquid that is then polymerized to a solid form to encapsulate the pigment. The pigment preferably has hydroxyl groups reactive with the polysiloxane. The slurry is spray dried and the coated pigment particles collected in a standard cyclone collector and air dried at 100° C. to complete the cure of the encapsulating polysiloxane film.
The abstract of WO 01/92359 discloses polymerically micro-encapsulated pigments for coatings prepared from a particle having an average diameter of 10 nm to 1 mm with a surface first reacted with a compound containing active groups of phosphoric acid, phosphonic acid, sulfonic acid sulfonate, amino or carboxylate groups, then the active groups bonded to the surface are reacted with an initiator containing a leaving group, and, finally, ATRP graft polymerization is carried out on the initiator with at least one olefinically unsaturated monomer.
Encapsulated pigments have been made for use in ink jet inks. U.S. Pat. No. 6,057,384 discloses an aqueous ink jet ink including a colorant associated with core-shell polymers. The monomers of a first polymer are selected to enhance adhesion to the colorant, while the monomers of a second polymer are selected to confer film-forming capability during drying and a durable film after drying. The polymers are associated with the colorant by direct milling in a liquid phase, as in the Examples in columns 26-35 or by a “hot stir” process with polymer and solvent of the first polymer and milling in a liquid phase with the second polymer. The milled pigments are diluted with water to prepare the final ink.
JP 2000 281951 describes dissolving or dispersing a colorant in an oil-soluble solvent, then emulsifying it in water. The emulsion and a resin are dissolved in a water-insoluble organic solvent and phase-inverted to an aqueous emulsion, providing colorant, a surfactant with ethylene oxide chains, and cationic resin-containing particles with average particle size 0.01 to 2.0 microns. The ink gives an image resistant to scratches, markers, and water. JP1111 6881 describes ink jet ink containing pigment encapsulated in a hydrophilic resin that is emulsifiable in water and a hydrophobic resin that is not emulsifiable in water. The ink does not clog ink jet nozzles and is resistant to weather and water. U.S. Patent Application No. 2003/097961 discloses milling together a polymer dissolved in organic solvent with a pigment, adding a crosslinker, emulsifying the mixture in water, removing the organic solvent, and, in the emulsified phase, crosslinking the polymer with the crosslinker. The product is said to be an emulsion of pigment enclosed in a crosslinked polymer. An ink is prepared by gradually adding an aqueous medium containing further ink components to the dispersion of the colorant.
These ink jet ink publications, however, do not address many issues that arise in industrial and automotive coatings, including the issues of forming a continuous, protective coating layer, smoothness of the coating layer, complex rheological behavior required during application and cure of the coating composition, and manufacturing concerns.
Thus, there remains a need for a method that would create an intermediate form of a colorant which would equalize or neutralize the effects of different pigments on the properties of the coating material. This allows the use of an intermediate with better handling and storage characteristics, capable of reducing the number of processing steps required in paint manufacturing.